An organic light-emitting device is a device in which a thin film containing a fluorescent organic compound or a phosphorescent organic compound is sandwiched between an anode and a cathode; an exciton of the fluorescent compound or the phosphorescent compound is produced by injecting an electron or a hole from each of the electrodes and the light radiated when the exciton returns to the ground state is utilized.
In a research by Eastman Kodak Company in 1987 (Appl. Phys. Lett. 51, 913 (1987)), there is reported a light emission of about 1,000 cd/m2 at an applied voltage of about 10 V for a device of separated-function two-layered structure using ITO for anode and a magnesium-silver alloy for cathode, respectively, an aluminum-quinolinol complex as an electron-transporting material and a light-emitting material and a triphenylamine derivative as a hole transporting material. Related patents include U.S. Pat. No. 4,539,507; U.S. Pat. No. 4,720,432 and U.S. Pat. No. 4,885,211.
In addition, light-emission from ultraviolet to infrared is possible by changing the type of fluorescent organic compounds and researches of various compounds have been conducted actively recently. For example, they are described in U.S. Pat. Nos. 5,151,629; 5,409,783; 5,382,477; 5,130,603; 6,093,864; 5,227,252; Japanese Patent Application Laid-Open No. H05-202356; Japanese Patent Application Laid-Open No. H09-202878 and Japanese Patent Application Laid-Open No. H09-227576.
In recent years, there have been a number of studies in which phosphorescent compounds are used as a light-emitting material and the energy in a triplet state is used for an EL emission. A group of Princeton University has reported that an organic light-emitting device using an iridium complex as a light-emitting material exhibits a high light-emitting efficiency (Nature 395, 151 (1998)).
Moreover, a group of Cambridge University has reported (Nature 347, 539 (1990)) an organic light-emitting device using a conjugated polymer other than the organic light-emitting device using monomeric materials as described above. In this report the light-emission in a monolayer is confirmed by forming a film of polyphenylenevinylene (PPV) in a coating system.
The related patents on organic light-emitting devices using conjugated polymers include U.S. Pat. Nos. 5,247,190; 5,514,878; 5,672,678; 5,317,169; 5,726,457 and Japanese Patent Application Laid-Open No. H05-247460.
Thus, recent progress in organic light-emitting devices is remarkable, and possibilities for a wide range of applications are indicated since it is characterized in that a thin and light-weight light-emitting device having high luminance at a low applied-voltage, diversity of light-emitting wavelength and high-speed response can be prepared.
However, a higher-luminance light output or high conversion efficiency is required under present circumstances. In addition, there are numbers of problems in terms of durability such as the variation with time during use for a long period of time and the deterioration due to an atmospheric gas containing oxygen or humidity. Moreover, the light-emission of blue, green and red having a good color purity is required for applications such as a full-color display, but these issues are not sufficiently satisfied.
Aromatic compounds and condensed polycyclic aromatic compounds have been studied in great numbers as fluorescent organic compounds to be used for an electron-transporting layer or a light-emitting layer. These include, for example, Japanese Patent Application Laid-Open No. H04-68076; Japanese Patent Application Laid-Open No. H05-32966; Japanese Patent Application Laid-Open No. H06-228552; Japanese Patent Application Laid-Open No. H06-240244; Japanese Patent Application Laid-Open No. H07-109454; U.S. Pat. No. 6,203,933; Japanese Patent Application Laid-Open No. H09-241629; U.S. Pat. No. 6,387,547; U.S. Pat. No. 6,399,223 and Japanese Patent Application Laid-Open No. 2000-268964. However, nothing that sufficiently satisfies light-emission luminance and durability has been obtained so far.